Hi Rich, just a hint first: this is an interesting question, but it's very USRP-centric; there's the usrp-users mailing list especially for USRP-related things, which I find quite interesting, most of the time [1] :)
The N210 is a modular USRP, meaning that the mixing is usually done on a daughterboard, which will usually actually take your input signal and bring it down to baseband. Therefore, the ADC always sees -50MHz to +50MHz of signal (it's a complex sampling ADC, hence the negative frequencies), and all daughterboards for the N2x0 are limited to a 40MHz baseband bandwidth, so there is no aliasing (^1). The N2x0 architecture _always_ uses the 100MHz sampling rate on RX, and the user is free to choose any integer fraction of that rate (^2). Using digital filtering and decimation, the 100MHz Nyquist bandwidth is reduced to the desired sampling rate. Thus, in usual operation, your daughterboard will be tuned, to generate a 300MHz (complex) tone, with which it mixes your signal down to -160kHz and 160kHz, which you will see with any sampling rate >320kHz. You are free to specify which frequency the daughterboard tunes to, so you can also tune to let's say 295MHz and see your signal at 5MHz +- 160kHz, although you'll need to use a much higher sampling rate, for example 100MHz/18~=5.56MHz, or your signal will be filtered out. I hope that answered your question :) Greetings, Marcus ^1 there's the exception of the basicRX daughterboard, which neither mixes nor has the 40MHz low pass characteristics; if you use that, you will have to make sure that your signal fits into the ADC's bandwidth, otherwise you *will* see images. ^2 as long as the resulting rate fits through gigabit ethernet, ie. 25MS/s for 16bit, 50MS/s for 8bit max. [1] http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com On 12/08/2014 08:23 PM, Richard Bell wrote: > I have a binary FSK signal centered at 300 MHz with 160 kHz offset peaks > from the center freak. Therefore the lowest signal frequency of interest is > 300-0.16 = 299.84 MHz and the highest is 300.16 MHz. > > I know the maximum sampling rate of the USRPN210 ADC is 100 MSPS. If the > USRP uses that sample rate of 100 MSPS, it will alias my signal onto itself > in an unwanted manor. > > My question is, how does the USRPN210 decide what sample rate to set the > ADC to? Is it a function of both the center frequency and the sample rate > setting of the UHD source block? > > Thanks, > Rich > > > > _______________________________________________ > Discuss-gnuradio mailing list > [email protected] > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
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